Invasive Plants Do Not Exert Univocal Responses on Soil Abiotic and Biotic Characteristics

Invasive plants are among the top five drivers of biodiversity loss, primarily due to competition and allelopathy. By releasing root exudates, they alter soil properties, influencing both the abiotic and biotic characteristics of soil. The effects of invasive plants on soil characteristics and biota remain underexplored, with findings on this topic often being controversial and context-dependent. This study aimed to understand the impact of two invasive species—black locust (Robinia pseudoacacia L.) and tree of heaven (Ailanthus altissima Mill.)—on soil abiotic characteristics, microbial and Collembola communities. Comparisons were made with soils under two types of native vegetation cover: holm oak (Quercus ilex L.) and herbaceous vegetation. In fall 2023, twelve sites within Vesuvius National Park (three per plant cover type) were sampled to assess soil characteristics, microbial biomass and activities, and Collembola communities. Tree of heaven increased soil pH (6.95), bacterial biomass (42.94 ng g−1) and Collembola density (2038 organisms m2) while reducing water content (10.6% d.w.) and organic carbon (1.21% d.w.). Black locust increased nitrogen content (0.70% d.w.) but reduced microbial biomass (22.85 ng g−1) and Collembola density (873 organisms m2). Tree of heaven soils showed a higher proportion of hemi-edaphic Collembola (48.3%) compared to black locust soils, which were dominated by eu-edaphic forms (42.2%). Despite these differences, Collembola species composition was poorly diversified under invasive plants, with Brachystomella parvula and Protaphorura armata dominating both types of cover. In conclusion, the presence of invasive plants was associated with declines in soil organism biodiversity, underscoring their disruptive influence on forest ecosystems.